FIGS. 3A and 3B show a conventional vacuum container of a fluorescent display tube (refer to Japanese Patent Application Publication No. 2003-68189). FIG. 3A is a perspective cross-sectional view of the vacuum container, and FIG. 3B is a cross-sectional view of the vacuum container taken along the line X1-X1 shown in FIG. 3A. The vacuum container is constituted of an anode substrate 11, a front substrate 12 and side plates 13. The anode substrate 11 is provided with a conductive film 14 formed thereon. The conductive film 14 includes an anode electrode made of a thin film forming a fluorescence emitting film and an anode wiring and the like. The vacuum container further includes a cathode electrode C which may be a thermal electron emitting filament. The anode substrate 11, the front substrate 12 and the side plates 13 are made of glass and are integrally bonded together using frit-glass (not shown). Generally, the substrate and the side plate used for the fluorescent display tube are made of soda-lime glass. However, the use of the soda-lime glass to form the anode substrate 11 provided with the thin conductive film 14 may cause a migration problem which leads to a short-circuit between the electrodes and the wirings of the conductive film 14. Therefore, the anode substrate 11 is generally made of high strain point glass in order to prevent the migration problem.
Furthermore, when forming the thin conductive film on the glass plate, it is desirable to use a thin glass plate to reduce the weight to facilitate the handling of the glass plate. Typically, the glass plate has thickness of about 1.8 mm. However, the glass plate having the thickness of about 1.8 mm is not strong enough for the use as the vacuum container of the fluorescent display tube. In view of this problem, Japanese Patent Application Publication No. H07-302559 proposes to provide a reinforcing glass plate bonded to the glass substrate provided with the thin conductive film. FIG. 3C is a cross-sectional view of an example of the conventional vacuum container provided with the anode substrate 11 provided with a reinforcing substrate 112. More specifically, the anode substrate 11 is constituted of a substrate 111 on which the conductive film 14 is formed (hereinafter referred to as a conductive film forming substrate) and the reinforcing substrate 112 bonded to the conductive film forming substrate 111. Using frit-glass 113 applied on the reverse surface of the conductive film forming substrate 111 (opposite to the surface of the substrate 111 on which the conductive film 14 in formed). In the anode substrate 11 fully covered with the frit glass 113 on the reverse surface of the conductive film forming substrate 111 shown in FIG. 3C, air bubbles present between the conductive film forming substrate 111 and the reinforcing substrate 112 cannot be removed or released outside completely when the frit-glass 113 is heated and melted. In addition, the space between the conductive film forming substrate 111 and the reinforcing substrate 112 does not become uniform, because the frit-glass does not spread 113 between the conductive film forming substrate 111 and the reinforcing substrate 112 in an uniform thickness.
In view of the problems relating to the anode substrate 11 explained hereinabove, the inventors of the present invention have proposed an anode substrate 21 provided with strip-shaped frit-glass layers FG applied on a conductive film forming substrate 211 as shown in FIG. 4. FIG. 4A shows a cross-sectional view of the vacuum container having the anode substrate 21, FIG. 4B shows a cross-sectional view of the vacuum container taken along the line X2-X2 shown in FIG. 4A, and FIG. 4C shows the vacuum container of FIG. 4A seen from the direction of the arrow X3 of FIG. 4A in which a reinforcing substrate 212 is eliminated for simplicity. FIG. 4C shows cracks created on the conductive film forming substrate 211.
The vacuum container of FIG. 4A is constituted of the anode substrate 21, the front substrate 22 and the side plates 23. The anode substrate 21 includes the conductive film forming substrate 211, the reinforcing substrate 212, and the strip-shaped frit-glass layers FG constituted of the rectangular strip-shaped frit-glass layers FG1 through FG11. The conductive film forming substrate 211 and the reinforcing substrate 212 are bonded together by means of the strip-shaped frit-glass layers FG1 through FG11. The frit-glass layers FG1 through FG11 are equal in length and arranged at a predetermined interval. Furthermore, the frit-glass layers FG1 through FG11 are arranged so that the respective distance between both longitudinal ends of the frit-glass layers to both transverse ends, namely both upper and lower ends of the conductive film forming substrate 211 shown in FIG. 4B are equal.
The vacuum container of FIGS. 4A and 4B can solve the problems in the vacuum container of FIGS. 3A through 3C by forming the strip-shaped frit-glass layers on the conductive film forming substrate. However, there is still a problem in the vacuum container of FIGS. 4A and 4B. That is, for the vacuum container of FIGS. 4A and 4B, the conductive film forming substrate 211 is made of an expensive glass plate with a high strain point, while the reinforcing substrate 212 is made of the inexpensive soda-lime glass plate in order to reduce the manufacturing cost of the vacuum container. As a result, when the vacuum container is heated and cooled during a sealing process of the vacuum container, cracks are created at the conductive film forming substrate 211 as shown in FIG. 4C. In FIG. 4C, the cracks are created at four locations 211C on the conductive film forming substrate 211 corresponding to the both longitudinal ends of the frit-glass layers FG1 and FG11, namely outermost the terminating ends of the frit-glass layers FG1 through FG 11 frit closest to the side plate 23.
The formation of the cracks is caused by the difference in the thermal expansion coefficient between the conductive film forming substrate 211 and the reinforcing substrate 212 due to excessive stress applied locally at the location 211C when the conductive film forming substrate 211 and the reinforcing substrate 212 having the different thermal expansion coefficient to each other are heated. Further to explanation regarding to the stress applied to the conductive film forming substrate 211 will be explained hereinafter. In this regard, the thermal expansion coefficient of the soda-lime glass is 93×10−7/degrees Celsius, the high strained point glass is 85×10−7/degrees Celsius and the frit-glass is 78×10−7/degrees Celsius.